BIRDS, THEIR STRUCTURE AND ADAPTATIONS 287 



the next stroke, a much easier task and one which does not 

 toughen it. 



Adaptations for Active Life. The act of flight requires more 

 work than any other form of locomotion. This is shown by the 

 enormous breast muscles that operate the wings, and the general 

 activity of the bird's whole life. Great amounts of energy are 

 required which means large food-getting and digestive ability. 

 This, in turn, demands a remarkably complete respiratory system 

 to provide for rapid oxidation and release of energy. 



Digestion. Birds are provided with a crop for storage, a gizzard 

 in which small stones take the place of teeth for chewing, and 

 very powerful digestive fluids, all of which work together to care 

 for the vast amount of fuel needed to run so powerful an engine. 

 A bird usually eats several times its own weight of food every day, 

 so the common expression to " have an appetite like a bird " is 

 hardly a suitable comparison for a light eater. 



Respiration. The respiratory organs consist of very finely 

 cellular lungs; behind these are the air sacs which hold the reserve 

 air and permit all the lung tissue to be used in supplying oxygen 

 to the blood. These air sacs also aid in this process. The rate of 

 respiration is very high and the normal temperature is from 102 

 to no degrees, which would be fatal to man and to most other 

 animals. Rapid oxidation means rapid production of waste 

 matters and these are removed largely by the very highly developed 

 lungs, there being little liquid urine eliminated by the kidneys, 

 and no sweat glands. Crystals of urea are excreted by the 

 kidneys. 



Not only do the lungs provide the blood with oxygen for oxida- 

 tion, and also remove waste, but in addition supply the air for 

 singing, of which many birds require a large amount. It might 

 be of interest to mention that the bird's song is not produced in 

 the throat, but at the base of the trachea where the tubes from each 

 lung join. Here is located the " song box," a very delicate and 

 highly adjustable structure. 



Circulation. To transport this large burden of digested food, 

 oxygen, and the waste products of oxidation, there is required a 



